Lining 'em up
With four drives, four controllers, and three different flash providers, there's a lot to keep track of in this latest wave of SSDs. To help make some sense of the collection assembled today, we've put together a quick comparison table that covers the basics.

As you can see, the drives are split between 64 and 128MB of cache. All but one packs 128GB of storage capacity, with the SiliconEdge Blue doubling that total. Each of these drives is, of course, available at different capacity points. Corsair and Plextor offer their drives in 64 and 128GB capacities. Western Digital has 64, 128, and 256GB flavors of the SiliconEdge, while Kingston takes its SSDNow range all the way up to 512GB.

Higher-capacity SSDs tend to offer better performance than lower-capacity models because they allow the drive controller to spread the load across additional chips. However, WD quotes identical performance figures for its 64, 128, and 256GB SiliconEdge Blue drives, and Kingston does the same for the different sizes of the V+. To take advantage of extra flash chips, a drive controller needs enough memory channels to use them effectively. The JMF612 may simply not have enough memory channels to exploit more than the eight chips necessary to make up a 64GB drive. By the same token, the Toshiba controller inside the SSDNow may not benefit from having more than the eight flash chips used on that 128GB model.

Rather than using capacity or chip counts to predict performance, we can get a sense of things by looking at the only common performance specifications published for all four drives: the maximum read- and write-speed ratings. Corsair has the lead on paper, with the Nova boasting faster ratings than the rest of the pack. The SSDNow and SiliconEdge sit in a sort of tie for second place; the Kingston drive claims faster writes, and the WD promises quicker reads. Then there's the Plextor, which looks set to read and write at least 100MB/s slower than the others.

We will, of course, take a closer look at transfer rates in a momentboth in directed tests and with real-world file copies. Before digging into our performance results, it's worth taking a moment to discuss less technical factors like pricing and warranty coverage.

Despite its anemic performance ratings and lack of TRIM support, the Plextor drive is listed at $399 at Newegg. It does have one more year of warranty coverage than the Nova, but Plextor's three-year coverage matches what's available from WD and Kingston. The PX-128M1S is going to have to pull off a few surprises to justify its price tag, because the free copy of Acronis True Image that's included in the box isn't going to be enough.

Obviously, the 256GB SiliconEdge Blue is the most expensive drive of the bunch. 128GB flavors sell for $400, so you're looking at the same cost per gigabyte one notch down on the capacity ladder. That said, even a 128GB SiliconEdge Blue will be more expensive than the Corsair Nova, which itself runs a good $50 more than the SSDNow V+. We'll look at each drive's actual cost-per-capacity a little later in the round-up.

TRIM at last
For all their blazing-fast performance, flash-based SSDs live with a crippling handicap: the block-rewrite penalty. Impossible to avoid, this penalty stems from the nature of flash memory itself. Flash cells are typically arranged in 512KB blocks made up of 4KB pages. Empty cells can be written to directly in 4KB chunks. However, if any pages within a cell are occupied, the entire block must be rewritten. A block rewrite is required even if just a single page's worth of data is being written to the block.

As you might've guessed, this block rewrite is the source of our performance penalty. An SSD must perform additional steps when writing to an occupied cell. The entire contents of that cell must be read into memory and modified, and then the entire block must be rewritten. Since occupied cells can only be written to in 512KB blocks, even a small 4KB write will incur a 512KB block rewrite. The performance lost due to the larger write and the additional read and modify steps is referred to collectively as the block-rewrite penalty.

The detrimental performance impact of the block-rewrite penalty is made worse by the way operating systems have traditionally dealt with deleted data. When a file is deleted in Windows XP or Vista, for example, the flash pages it occupied are marked as available but are not actually cleared. The pages may be available in the sense that they no longer contain data that's considered valid. However, the data is still there, and thus the page is still occupied.

As more data is written to a solid-state drive over time, the SSD will eventually exhaust its supply of empty pages. After it reaches this used state, each and every subsequent write request will be slowed by the block-rewrite penalty, regardless of how much free space the drive reports to the operating system.

The TRIM command battles the block-rewrite penalty by reducing the number of unnecessarily occupied flash pages on a drive. When a file is deleted, the TRIM command lets a compatible SSD know that the associated pages are not just available, but safe to be erased. The drive is then free to clear those pages at its leisure.

Why not force pages to be erased immediately? Because flash memory, and in particular the MLC type found in consumer solid-state drives, can only endure a limited number of write-erase cycles. Forcing an SSD to clear the pages immediately might reduce the lifespan of the drive. Instead, TRIM leaves an SSD free to clear pages opportunistically, perhaps in conjunction with its own internal garbage-collection and wear-leveling mechanisms.

The TRIM command is currently supported by Windows 7, Windows Server 2008 R2, and the 2.6.33 Linux kernel released in February of this year. Microsoft hasn't detailed plans to port TRIM back to older versions of Windows, nor has Apple announced TRIM support for any version of OS X. You'll want to make sure that your motherboard's storage controller drivers support the command, too. Some may not, and others do only with restrictions. For example, Intel's latest Rapid Storage Technology drivers support TRIM for SSDs as long as they're running as single drives, but not if they're members of a RAID array.